1. Field of the Invention
The present invention relates to a sheet feeder. More particularly, the present invention relates to a sheet feeder provided with a feed roller for feeding a sheet.
2. Description of the Related Art
Conventionally, various sheet feeders provided with a feed roller for feeding a sheet are known (see JP-A-61-197349, JP-A-6-64981 and JP-A-10-139235).
JP-A-61-197349 discloses a sheet feeder which prevents the feeding of a sheet from being obstructed by the fact that the end of the curled sheet bumps into a feed roller by providing an auxiliary ring which smoothly introduces the end of the sheet to the contact part of a pair of feed rollers for feeding the sheet.
Furthermore, JP-A-6-64981 discloses a sheet feeder which eliminates rattle of a feed roller to a drive shaft while the feed roller is rotated by engaging an engaging hole having a substantial D-shaped cut of a fixing member attached to both side surfaces of the feed roller with a drive shaft having a sectional shape of a D-shaped cut.
Furthermore, JP-A-10-139235 discloses a sheet feeder which can reduce the number of parts by integrally forming a V-shaped elastic rib energizing a discharge roller in the direction pressing to the feed roller to the discharge roller compared with the case where the discharge roller and a member for energizing the discharge roller are separated.
FIG. 8 is a side view showing the structure of a conventional sheet feeder provided with a feed roller for feeding a sheet. FIG. 9 is an enlarged side view of the first intermediate gear of the sheet feeder according to the conventional example shown in FIG. 8. FIG. 10 is an enlarged side view of the second intermediate gear of the sheet feeder according to the conventional example shown in FIG. 8. Referring to FIG. 8, a feed roller 101 for feeding a sheet 200 is provided on a sheet feeder according to a conventional example. The feed roller 101 is provided with a rotary shaft 110a. A feed roller gear 101b is fixed to the rotary shaft 101a of the feed roller 101. A pinch roller 102 which presses the sheet 200 to the feed roller 101 is provided above the feed roller 101.
Furthermore, a first intermediate gear 103 for rotating the feed roller gear 101b is engaged with the feed roller gear 101b. As shown in FIG. 9, a circular bearing 103a is provided on the first intermediate gear 103. A circular bearing engaging part 104a of a first intermediate gear support shaft 104 which rotatably supports the first intermediate gear 103 is inserted into the circular bearing 103a of the first intermediate gear 103. The circular bearing 103a of the first intermediate gear 103 comes in contact with one position (P1) of the circumference of the circular bearing engaging part 104a of the first intermediate gear support shaft 104.
Furthermore, as shown in FIG. 8 and FIG. 10, a second intermediate gear 105 for rotating the first intermediate gear 103 is provided. The second intermediate gear 105 is provided with a small-diameter gear 105a engaged with the first intermediate gear 103 and a large-diameter gear 105b having a larger diameter than that of the small-diameter gear 105a. As shown in FIG. 10, a circular bearing 105c is formed on the second intermediate gear 105. A bearing engaging part 106a of a second intermediate gear support shaft 106 which rotatably supports the second intermediate gear 105 is inserted into the circular bearing 105c of the second intermediate gear 105. The circular bearing 105c of the second intermediate gear 105 comes in contact with one position (P2) of the circumference of the circular bearing engaging part 106a of the second intermediate gear support shaft 106.
Furthermore, as shown in FIG. 8, a driving transmission gear 107 for rotating the second intermediate gear 105 is engaged with the large diameter gear 105b of the second intermediate gear 105. The driving transmission gear 107 is fixed to a drive shaft 108a of a motor 108.
Next, referring to FIG. 8 through FIG. 10, the operation of the sheet feeder according to the conventional example showed in FIG. 8 will be described. As shown in FIG. 8, when the motor 108 is driven, the driving transmission gear 107 attached to the drive shaft 108a of the motor 108 is rotated in the direction of an arrow A shown in FIG. 8. The second intermediate gear 105 is rotated in the direction of an arrow B shown in FIG. 8 based on the rotation of the driving transmission gear 107.
In this case, as shown in FIG. 10, the second intermediate gear 105 accepts a force F2 as a resultant force of a force F3 applied from the driving transmission gear 107 and a force F4 applied as a drag from the first intermediate gear 103 when the first intermediate gear 103 is rotated. Thereby, since the circular bearing 105c of the second intermediate gear 105 is pressed to the circumference of the bearing engaging part 106a of the second intermediate gear support shaft 106 on the line of the force F2, frictional force of .F2 (. is coefficient of dynamic friction) acts between the bearing engaging part 106a and the bearing 105c of the second intermediate gear 105. As shown in FIG. 10, the contact position of the bearing 105c to the bearing engaging part 106a is moved to the position of P2 sloping at only an angle .3 to the rotating direction (the direction of the arrow B as shown in FIG. 10) of the second intermediate gear 105 along the circumference of the bearing engaging part 106a from the line of the force F2 by the frictional force .F2. The slope of the angle .3 prevents the contact point from being moved any further. That is, the frictional force .F2 for moving the contact point and a force for preventing movement at the sloping position of the angle .3. Thereby, the bearing 105c of the second intermediate gear 105 is rotated while coming in contact with the contact position P2 of the circumference of the bearing engaging part 106a of the second intermediate gear support shaft 106.
Next, the first intermediate gear 103 is rotated in the direction of an arrow E as shown in FIG. 8 based on the rotation of the second intermediate gear 105. In this case, as shown in FIG. 9, the first intermediate gear 103 accepts a force F1 as a resultant force of a force F5 applied from the second intermediate gear 105 and a force F6 applied as a drag from a feed roller gear 101b when the feed roller gear 101b is rotated. Thereby, since the circular bearing 103a of the first intermediate gear 103 is pressed to the circumference of the bearing engaging part 104a of the first intermediate gear support shaft 104 on the line of the force F1, the frictional force of .F1 (. is coefficient of dynamic friction) acts between the bearing engaging part 104a and the bearing 103a of the first intermediate gear 103. As shown in FIG. 9, the contact position of the bearing 103a to the bearing engaging part 104a is moved to the position of P1 sloping at only an angle .4 to the rotating direction (the direction of the arrow E as shown in FIG. 9) of the first intermediate gear 103 along the circumference of bearing engaging part 104a from the line of the force F1 by the frictional force .F1. The slope of the angle .4 prevents the contact point from being moved any further. That is, the frictional force .F1 for moving the contact point and a force for preventing movement at the sloping position of the angle .4. Thereby, the bearing 103a of the first intermediate gear 103 is rotated while coming in contact with the contact position. P1 of the circumference of the bearing engaging part 104a of the first intermediate gear support shaft 104.
The feed roller gear 101b is rotated in the direction of an arrow H shown in FIG. 8 based on the rotation of the first intermediate gear 103, and thereby the feed roller 101 is also rotated in the direction of the arrow H shown in FIG. 8. Thereby, the sheet 200 pressed to the feed roller 101 by a pinch roller 102 is fed in the direction of an arrow I shown in FIG. 8.
However, in the conventional sheet feeder shown in FIG. 8, the frictional force .F1 which acts between the bearing 103a of the first intermediate gear 103 and the circumference of the bearing engaging part 104a of the first intermediate gear support shaft 104 increases and decreases according to the fluctuation of the force F1 (see FIG. 9) that the first intermediate gear 103 accepts by the rotation. Thereby, the contact position P1 of the bearing 103a to the bearing engaging part 104a is inconveniently moved in a lateral direction along the circumference of the bearing engaging part 104a. Specifically, when the frictional force .F1 which acts between the bearing 103a and the circumference of the bearing engaging part 104a is larger by the increase of the force F1 applied by the rotation, the contact position P1 is inconveniently moved in the direction in which the angle .4 shown in FIG. 9 is larger. On the other hand, when the frictional force F1 which acts between the bearing 103a and the circumference of the bearing engaging part 104a is smaller by the decrease of the force F1 applied by the rotation, the contact position P1 is inconveniently moved in the direction in which the angle .4 shown in FIG. 9 is smaller. When the contact position P1 is moved, the first intermediate gear 103 is moved along the circumference of bearing engaging part 104a of the first intermediate gear support shaft 104 without being rotated. Thereby the rotational amount of the first intermediate gear 103 is fluctuated.
The frictional force .F2 which acts between the bearing 105c of the second intermediate gear 105 and the circumference of the bearing engaging part 106a of the second intermediate gear support shaft 106 increases and decreases according to the fluctuation of the force F2 (see FIG. 10) that the second intermediate gear 105 accepts by the rotation. Thereby, the contact position P2 of the bearing 105c to the bearing engaging part 106a is inconveniently moved in a lateral direction along the circumference of the bearing engaging part 106a. Specifically, when the frictional force .F2 which acts between the bearing 105c and the circumference of the bearing engaging part 106a is larger by the increase of the force F2 applied by the rotation, the contact position P2 is inconveniently moved in the direction in which the angle .3 shown in FIG. 10 is larger. On the other hand, when the frictional force .F2 which acts between the bearing 105c and the circumference of the bearing engaging part 106a is smaller by the decrease of the force F2 applied by the rotation, the contact position P2 is inconveniently moved in the direction in which the angle .3 shown in FIG. 10 is smaller. When the contact position P2 is moved, the second intermediate gear 105 is moved along the circumference of bearing engaging part 106a of the first intermediate gear support shaft 106 without being rotated. Thereby the rotational amount of the second intermediate gear 105 is fluctuated.
As described above, since the fluctuation of the rotational amounts of the first intermediate gear 103 and second intermediate gear 105 causes fluctuation of the rotational amount of the feed roller gear 101b, the feeding unevenness of the sheet 200 is generated. The larger the number the intermediate gear is, the larger the fluctuating amount accumulated is, and thereby the feeding unevenness of the sheet 200 is also larger. As a result, it is difficult that the feeding of the sheet 200 due to the feed roller 101 is accurately controlled.
Conventionally, to control the feeding unevenness of the sheet 200, the gap between the bearing 103a of the first intermediate gear 103 and the bearing engaging part 104a of the first intermediate gear support shaft 104, and the gap between the bearing 105c of second intermediate gear 105 and the bearing engaging part 106a of the second intermediate gear support shaft 106 are reduced to as small a volume of material as possible. Therefore, it is necessary to improve the accuracy of parts, and there is a problem that part costs rise as a result.
In the sheet feeders disclosed in JP-A-61-197349, JP-A-6-64981 and JP-A-10-139235, it is difficult to accurately control the feeding of the sheet due to the feed roller as well as the conventional sheet feeder shown in FIG. 8 since no countermeasure for suppressing the fluctuation of the rotational amount of the gear for transmitting the rotation to the feed roller is performed.